Embodiments presented herein relate generally to electrical equipment monitoring and more specifically to transformer monitoring.
Power transformers are extensively used in electrical power generation and distribution systems to efficiently control the generation and distribution of power. With the need of power increasing by the day, the size of power transformers both in terms of power rating and voltage rating has increased remarkably. Transformers rated as high as hundreds of MVA are available.
A transformer typically includes a magnetic core made of a ferromagnetic material as well as primary and secondary windings wound over the magnetic core. The windings of the transformer are insulated to prevent short circuits between successive turns. Prolonged usage of the transformer may degrade the winding insulation and lead to short circuits. A short circuit tends to create circulating currents which in turn results in local hot spots within the transformer. If not detected and corrected in a timely manner, such hot spots may ultimately lead to a transformer failure and a power outage condition.
One method for determining transformer health is to subject a transformer to high voltage pulses and compare the transformer's response to the response of a healthy transformer. Another method to determine transformer health is to evaluate power dissipation in the transformer under open circuit conditions at various input voltages. However, such controlled excitation based methods either require the transformer to be removed from the power system for testing or require use of redundant transformers so that one transformer may be used while another is being tested. It is undesirable to have to turn off a power system during such testing. The increased cost associated with using redundant transformers is also undesirable.
One online method for monitoring transformer health that does not require redundant transformers uses a differential relay to sense a difference between the primary current and the secondary current. Under fault conditions the difference deviates from a baseline. A substantial deviation in the difference indicates the presence of a fault within the transformer. However, the differential relays are not sensitive to low level faults as they are tuned to overlook the momentary difference of current when the primary winding is energized. Thus, the differential relay protection systems detect only the faults which are substantially acute.